Devices and methods for implanting fusion cages

ABSTRACT

Methods and instrumentation for spinal interbody fusion are disclosed. The instruments and methods are particularly adapted for interbody fusion in an unreamed disc space. One instrument is a distractor assembly including a first distractor and a second distractor configured to be inserted in side-by-side relation in the disc space. Fusion cages adapted for insertion into an unreamed disc space are provided to enhance load distribution between adjacent vertebral bodies and lateral stability of the spinal column. The fusion cages may be inserted after distracting the disc space with the distractor assembly. Instruments for preparing the disc space through the inserted fusion cages or for use with the distractor assembly are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing date ofProvisional application Ser. No. 60/150,787, filed Aug. 26, 1999,entitled DEVICES AND METHODS FOR IMPLANTING FUSION CAGES. The referencedapplication is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to surgical procedures forspinal stabilization, and more specifically to instrumentation andtechniques for inserting a spinal implant within the intervertebraldisc-space between adjacent vertebra. More particularly, while aspectsof the present invention may have other applications, the inventionprovides instruments, techniques, and implants especially suited forimplanting one or more fusion cages in an unreamed disc space.

There have been an extensive number of attempts to develop anexceptional intradiscal implant that could be used to maintain theheight and stability of the disc interspace between adjacent vertebra,at least until complete arthrodesis is achieved. These “interbody fusiondevices” have taken many forms. For example, one of the more prevalentdesigns takes the form of a cylindrical implant. These types of implantsare presented in patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan,U.S. Pat. No. 4,878,915; Ray, U.S. Patent Nos. 4,961,740 and 5,055,104;and Michelson, U.S. Pat. No. 5,015,247. In the cylindrical implants, theexterior portion of the cylinder can be threaded to facilitate insertionof the interbody fusion device, as represented by the Ray, Brantigan andMichelson patents. In the alternative, some of the fusion implants aredesigned to be driven into the intradiscal space with little or norotation. For example, this type of device is represented in the patentto Brantigan. A combination implant having the ability for threadedinsertion or push-in insertion is disclosed in U.S. Pat. No. 5,782,919to Zdeblick et al.

U.S. Pat. No. 5,484,437 to Michelson discloses a technique andassociated instrumentation for inserting a fusion device. As describedin more detail in the '437 patent, the surgical technique involves theuse of a distractor having a penetrating portion that urges thevertebral bodies apart and a hollow sleeve having teeth at one end thatare driven into the vertebrae adjacent the disc space created by thedistractor. These teeth engage the vertebra to maintain the disc spaceheight during subsequent steps of the procedure following removal of thedistractor. In accordance with one aspect of the '437 patent, a drill ispassed through the hollow sleeve to remove portions of the disc materialand vertebral bone in order to ream the disc space and reduce theendplates to bleeding to produce a prepared bore for insertion of thefusion device. The drill is then removed from the sleeve and a fusiondevice having a diameter greater than the height of the disc space ispositioned within the disc space using an insertion tool.

The device depicted in the Michelson patent is representative of thistype of hollow implant which is typically filled with a bone growthinducing substance to promote bone growth into and through the device.This implant includes a plurality of circular apertures whichcommunicate with the hollow interior of the implant, thereby providing apath for tissue growth between the vertebral end plates and the bonegrowth material within the implant.

One problem that is not adequately addressed by the above prior devicesconcerns restoring and maintaining the normal anatomy of the fusedspinal segment. Naturally, once the disc is removed, the normal lordoticor kyphotic curvature of the spine may be altered. In response to thisproblem, the adjacent vertebral bodies may be reamed with a cylindricalreamer that fits the particular shape of the implant. In some cases,distraction techniques are used to establish the normal curvature priorto reaming. However, for a cylindrical implant, the over-reaming of theposterior portion is generally not well accepted because of theextensive removal of load bearing bone of the vertebrae. Over time, theimplant tends to migrate into the vertebral bodies since theload-bearing surfaces of the endplates are no longer adequate for theimplant to support the spinal column loads. This migration is oftenreferred to as subsidence. When an implant subsides into adjacent bone,the disc space can collapse, resulting in potentially adverseconsequences to the patient's health.

Another problem is that when the disc space and adjacent endplates arereamed, the implant must have a height greater than that of the originaldisc space height to restore the disc space to its normal anatomy. Whenlarge implants are bi-laterally inserted in the disc space, the lateralspacing and separation between the implants that can be attained is lessthan that attainable with smaller implants used in the same disc space.This positions the larger implants closer to the medial portion of thedisc space and vertebral endplates, thus increasing the risk ofmigration into the vertebral bodies and subsidence of the spinal columnaround the implant. Also, lateral stability of the spinal column isreduced since less support is provided at the hardy bony peripheral ringof the adjacent vertebral bodies. Thus, it is desirable to maintainproper lateral separation of the implants in the disc space so that eachimplant is bearing on the strongest portion of the vertebral bodies andthe lateral stability of the spinal column is maintained.

While the more recent techniques and instrumentation represent anadvance over earlier surgical procedures for the preparation of the discspace and insertion of the fusion device, the need for improvement stillremains. There remains a need for interbody fusion cages that may beinserted into an unreamed disc space, as well as instruments andtechniques for inserting these fusion cages in an unreamed disc space tostabilize the spine. The present invention is directed to these needsand provides convenient methods, instruments, and implants for effectivepreparation of an unreamed disc space in conjunction with implantplacement.

SUMMARY OF THE INVENTION

One object of the present invention is to provide instruments permittingplacement of a fusion cage in an unreamed disc space. One instrument ofthe present invention includes a first distractor and a seconddistractor configured to be inserted in side-by-side relation in thedisc space. At least one distractor has a guide surface abutting theother distractor to maintain the distractor spacing as the distractorpair is inserted into the disc space. In a preferred form, the guidesurface is offset from the central axis of the instrument.

In one specific embodiment, each distractor has a body portion with aleading end and a trailing end. The body portions distract the discspace and form a channel therethrough as the distractors are inserted.Preferably, at least one of the distractors is provided with a mediallyextending portion extending from the body portion towards medial area ofthe disc space. The guide surface is formed by a medial side of themedially extending portion. The medially extending portion can beprovided with a height less than that of the body portion. The guidesurface of the medially extending portion guides the insertion of a cageinto the distraction channel formed through the unreamed disc spaceremaining after withdrawal of the other distractor.

In another instrument of the present invention, first and seconddistractors are provided with first and second central spacers. Eachcentral spacer has a width between its guide surface and the bodyportion of the distractor from which it extends. The central spacersextend medially from the body portion into the disc space so that theguide surfaces are adjacent one another. In one form, the first centralspacer has a width that is greater than the width of the second centralspacer. The guide surfaces maintain the spacing between the bodyportions as the first and second distractors are inserted into the discspace. Alternatively, the central spacers are provided with an equalwidth. In another form, the first and second distractors each furtherinclude a lateral spacer having a width that tapers from a maximum widthat the trailing end of the body portion to a minimum width at the distalend of the body portion. It is preferred that the body portion have adiameter that is substantially the same as the diameter of the leadingend of the fusion cage to be inserted into the disc space. In onepreferred form, the guide surface of the central spacer of the firstdistractor guides the insertion of an implant into the distractionchannel formed through the unreamed disc space remaining afterwithdrawal of the second distractor.

In yet another specific embodiment of the distractors of the presentinvention, the leading end of the body portion is tapered to facilitateinsertion of the distractor into the unreamed disc space. Alternatively,the leading end of the body portion is rounded. It is also contemplatedthat the top and/or bottom surfaces of the body portion contacting thevertebral endplates may be roughened along a portion of the length ofthe body portion starting at the trailing end. The roughened surfacescrapes the vertebral endplate during insertion and resists migration ofthe distractors in the disc space. The body portions of the distractorscan also include a hollow threaded cylindrical hole or bore to connectthe distractor to an obturator or shaft used for inserting andwithdrawing the distractors.

In another aspect of the present invention, a method of distracting adisc space is provided. The method comprises providing a firstdistractor having a first longitudinal axis and a guide surface spaced afirst distance from the first axis, and providing a second distractorhaving a second longitudinal axis. Each distractor is connected with acorresponding shaft, and positioned with the guide surface abutting aside of the second distractor. The first and second distractors aresimultaneously inserted to distract the disc space to form a distractionchannel.

In one form, the method additionally includes preparing a starterchannel at the anterior lip of the disc space adjacent the firstdistractor. A channel starter instrument is provided with an outer shaftand an inner shaft. A cutting blade is positioned between the outershaft and the inner shaft. The inner shaft has an end portion receivedwithin an opening formed at the trailing end of the first distractor.The cutting blade removes a portion of the endplate thickness at theanterior lip of the vertebral bodies, thus forming a starting channel inthe disc space that is coextensive with the channel formed in the discspace by the removed second distractor. The above steps are repeated atthe location of the first removed distractor if desired to form a secondstarter channel.

Yet a further aspect of the present invention provides a method forinserting one or more fusion cages in an unreamed disc space afterinsertion of the distractors as described above. The second distractoris removed from the disc space, forming a distraction channel in thedisc space. A fusion cage, preferably having a root diameter or heightapproximating the disc space height, is inserted in the disc space. Thefirst distractor guide surface maintains lateral positioning of the cagein the disc space as it is inserted. The first distractor is removed,and a spacer device is secured to the first fusion cage to act as aguide for insertion of a second fusion cage in the distraction channelformed by the first distractor.

In another form, the first and second fusion cages have openings intheir top and bottom surfaces adjacent the endplates. A curette or othercutting instrument is placed into the cage, and the bone portion of thevertebral endplates adjacent the openings is removed. The remainingportion of the endplates in contact with the top and bottom surfaces ofthe cages remains intact to provide a strong bearing surface. Bonegrowth material is then placed within the fusion cages.

Yet another aspect of the present invention is to provide a fusion cagefor insertion in an unreamed disc space. The cage includes a body havinga hollow interior extending between a trailing end and a leading end. Inone form, the body is threaded and tapered to restore lordosis wheninserted in the disc space. The cage has a top surface and a bottomsurface positioned in contact with the intact endplates of the vertebraewhen the cage is implanted. The body defines a number of openings in thetop and bottom surface. Preferably, external threads extend outwardlyfrom the body portion and engage the cage to the bony end plates andharvest disc material and bone from the endplates for deposit throughthe openings into the hollow interior. In another form, the fusion cageis provided with threads with a swept back profile that increase indepth from the leading end to the trailing end to prevent backout of theinserted cage from the disc space.

One aspect of the invention contemplates providing an interbody fusioncage or device having opposed upper and lower bearing surfaces separatedby a height. In one form, the height tapers along the length of thedevice to match angulation between endplates of adjacent vertebra. In apreferred form, the device includes migration resistance structuresintended to limit movement of the fusion device in the disc space.Preferably, these migration resistance structures may include threads,ridges, knurling, spikes, or other surface irregularities extending fromthe bearing surface. One improvement of interbody fusion devicesaccording to the present invention can be characterized as the spacingdistance being substantially equal to the distance between unreamedendplates of adjacent vertebra thereby eliminating the need for removingvertebral endplate bone to form an insertion channel.

In another aspect of the invention, there is provided a method forpreparing vertebral endplates through a fusion cage inserted in a discspace. The fusion cage includes at least one opening communicating withthe endplate. Bone is removed from the endplate through the at least oneopening after insertion of the fusion cage into the disc space.

In one preferred form, the fusion cage is inserted into a disc spacehaving intact endplates. In another preferred form, a cutting instrumentis provided that is inserted through the cage and configured to removeendplate bone material through the at least one opening. In one form,the cutting instrument includes a burr for removing bony material. Inanother form, the cutting instrument includes a curette for removingbony material. In yet another form, the cutting instrument is configuredto remove bony material simultaneously from laterally adjacent holesthrough the fusion cage. In another preferred form, the fusion cageincludes a guide at the trailing end of the cage opposite the at leastone hole for the maintaining the cutting instrument alignment andfacilitating use of the cutting instrument in the fusion cage.

Still a further object of the present invention is to provide a spinaldisc space distractor assembly. The distractor assembly has a centralaxis extending therethrough. Preferably, the assembly includes a pair ofdistractors positioned in side-by-side relation, the distractors forminga guide surface therebetween. Preferably, the guide surface is offset adistance from the central axis.

The present invention also contemplates a method of preparing a discspace and inserting an implant in an unreamed disc space. The methodutilizes one or more of the instruments and implants described above toprepare the disc space for receiving an implant.

Related aspects, features, forms, embodiments, objects and advantages ofthe present invention will be apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a distractor assembly according to oneaspect of the present invention.

FIG. 2 is an end view of the distractor assembly of FIG. 1.

FIG. 3 is a partial top plan view of a shaft assembly connectable to thedistractor assembly of the present invention.

FIG. 4 is a top plan view of a third distractor usable with thedistractor assembly of FIG. 1.

FIG. 5(a) is a side elevational view of a further embodiment of adistractor of the distractor assembly of FIG. 1.

FIG. 5(b) is a side elevational view of another embodiment of adistractor of the distractor assembly of FIG. 1.

FIG. 6 is an end elevational view of yet a further embodiment of adistractor of the distractor assembly of FIG. 1.

FIG. 7 is a perspective view of another embodiment distractor assemblyaccording to the present invention.

FIG. 8 is a left end elevational view of the distractor assembly of FIG.7.

FIG. 9 is a right end elevational view of the distractor assembly ofFIG. 7.

FIG. 10 is a partial sectional view taken along the spinal column of asegment of the spinal column.

FIG. 11 is a partial cross-sectional end view illustrating prior artfusion devices inserted into a disc space.

FIG. 12 is a partial cross-sectional end view illustrating fusiondevices according to the present invention inserted into the disc space.

FIG. 13 is a side elevational view of a fusion cage according to anotheraspect of the present invention.

FIG. 13a is an enlarged cross-sectional view of a portion of the threadof the cage of FIG. 13.

FIG. 14 is a perspective view from the leading end of the fusion cage ofFIG. 13.

FIG. 15 is an end elevational view of the trailing end of the fusioncage of FIG. 13.

FIG. 16 is a perspective view from the trailing end of the fusion cageof FIG. 13.

FIG. 17 is a top plan view of the fusion cage of FIG. 13.

FIG. 18 is a side elevational view of a fusion cage according to anotheraspect of the present invention.

FIG. 19 is a perspective view from the leading end of the fusion cage ofFIG. 18.

FIG. 20 is an end elevational view of the trailing end of the fusioncage of FIG. 18.

FIG. 21 is a perspective view from the trailing end of the fusion cageof FIG. 18.

FIG. 22 is a top plan view of the fusion cage of FIG. 18.

FIG. 23 is a perspective view of a spacer device and fusion cageaccording to another aspect of the present invention before the spacerdevice is secured to a fusion cage.

FIG. 24 is a left end elevational view of the spacer device of FIG. 23.

FIGS. 25(a)-25(h) show various steps of a method according to thepresent invention using instruments and cages of the present invention.

FIG. 26 is a side elevational view of a fusion cage according to anotheraspect of the present invention.

FIG. 27 is a top plan view of the fusion cage of FIG. 26.

FIG. 28 is a top plan view of another embodiment spacer device accordingto the present invention.

FIG. 29 is a side elevational view of the spacer device of FIG. 28.

FIG. 30 is an end elevational view of the trailing end of the fusioncage of FIG. 26 with the spacer device of FIG. 28 connected thereto.

FIG. 31 is an end elevational view of the trailing end of an alternateembodiment of the fusion cage of FIG. 26.

FIG. 32 is a top plan view elevation of a fusion cage according to yetanother aspect of the present invention.

FIG. 33 is sectional view through line 33—33 of FIG. 32.

FIG. 34 is an end elevational view of the trailing end of the fusioncage of FIG. 32.

FIG. 35 is a side elevational view of a surgical instrument according toanother aspect of the present invention.

FIG. 36 is a top plan view of an alternate embodiment of the surgicalinstrument of FIG. 35.

FIG. 37 is a partial elevational view of a surgical instrument accordingto yet another aspect of the present invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any such alterations and furthermodifications in the illustrated devices, and any such furtherapplications of the principles of the invention as illustrated thereinare contemplated as would normally occur to one skilled in the art towhich the invention relates.

The present invention relates generally to instruments, devices, andmethods for performing vertebral interbody fusion. While it should beunderstood that the instruments and devices disclosed herein have manyuses, it is particularly contemplated that they may be used to performvertebral interbody fusion in an unreamed disc space with the endplatesremaining completely or substantially intact. It is also particularlycontemplated that the methods and instruments may be used in “open” ornon-laparoscopic procedures. It is also contemplated that suchprocedures may be completed without requiring the use of cannulas orguide tubes, and that cages may be placed in the disc space usingfreehand techniques and other instruments known in the art. However, theinstruments, methods and devices may be used and adapted if necessary,as known in the art, for use with guide tubes and laparoscopicprocedures.

Referring now to FIGS. 1-2, there is shown a distractor assemblyaccording to one aspect of the present invention. Distractor assembly 10may be coupled to one or more obturators or shaft assemblies, asdescribed in detail below with respect to FIG. 3, for insertion ofdistractors 12 a and 12 b into a disc space between the endplates ofadjacent vertebra in order to restore the proper disc space height priorto insertion of a fusion cage or other implant therein. Distractorassembly 10 has a central axis A and includes a first distractor 12 aand a second distractor 12 b positioned adjacent first distractor 12 a.Distractors 12 a and 12 b have central axes A1 and A2 separated by alateral spacing S.

Distractor 12 a includes a body portion 14 a around axis A1. Bodyportion 14 a extends between a leading end 22 a and a trailing end 24 a,and preferably defines hollow opening 20 a at trailing end 24 a.Extending laterally from body portion 14 a is lateral spacer 16 a, andextending medially from body portion 14 a is central spacer 18 a.Central spacer 18 a defines a guide surface 26 a adjacent seconddistractor 12 b. Second distractor 12 b includes a body portion 14 baround axis A2, and preferably defines hollow opening 20 b at trailingend 24 b. Body portion 14 b extends between a leading end 22 b and atrailing end 24 b. Extending laterally from body portion 14 b is lateralspacer 16 b, and extending medially from body portion 14 b is centralspacer 18 b. Central spacer 18 b defines a guide surface 26 b adjacentto and coextensive with guide surface 26 a of first distractor 12 a. Itis contemplated that guide surfaces 26 a and 26 b abut one another whendistractor assembly 10 is inserted into the disc space, but have beenillustrated as slightly offset in FIGS. 1-2 for the purposes of clarity.The components of first and second distractors 12 a and 12 b may berecited herein collectively by referring to, for example, body portion14 or lateral spacer 16.

It is further contemplated that a shaft assembly such as that shown inFIG. 3 may be provided with distractor assembly 10. Shaft assembly 40that includes obturators or shafts 41 a and 41 b. Shaft 41 a definesaxis L1 and includes a threaded portion 42 a to connect to a the shaftto a distractor. A shoulder 44 a is configured to abut against trailingend 24 a when distractor 12 a is connected to shaft 41 a with axes A1and L1 coextensive. Threaded portion 42 a is preferably threaded, andmates with corresponding threads formed in opening 20 a. Shaft 41 a alsoincludes a cam 43 a having a cam surface 45 a along shaft 41 b. Shaft 41b defines an axis L2 and includes a threaded portion 42 b. A shoulder 44b is configured to abut against trailing end 24 b when distractor 12 bis connected to shaft 41 b with axes A2 and L2 coextensive. Whilesportions 42 b are preferably threaded, it is understood that any meansknown to those skilled in the art may be employed to connect the shaftto its corresponding distractor, and that the shafts need not beremovable.

To insert the distractors 12 a and 12 b in the disc space, eachdistractor 12 a and 12 b is connected with the corresponding connectingportion 42 a and 42 b. Cam 43 a maintains the lateral separation andrelative alignment of shafts 41 a and 41 b as shaft 41 b abuts againstcam surface 45 a. In an alternate embodiment, shaft 41 a does notinclude cam 43 a, but the shaft 41 a has a lateral dimension sized tomaintain contact and alignment between shafts 41 a and 41 b. Thedistractors 12 a and 12 b are simultaneously driven into the disc spaceand positioned using an impactor cap and hammering techniques applied tothe proximal end of the shafts 41 a and 41 b as is know in the art.Lateral spacers 16 guide the distractor assembly 10 into the center ofthe disc space to ensure the distractor assembly 10 is in the properlocation.

In one preferred embodiment, the distractor 12 has a knurled orroughened surface 28 on the top and on the bottom of body portion 14adjacent the vertebral endplates. For ease of insertion, it is preferredthat the surface of body portion 14 be substantially smooth adjacentleading end 22 and that roughened surface 28 not extend to leading end22. In one specific embodiment, this smooth surface extends about 10millimeters from leading end 22 towards trailing end 24. Knurled orroughened surface 28 can be made by teeth or any combination or patternof indentations and projections formed on the surface of body 14. Eachroughened surface 28 scrapes or removes a portion of bone material fromthe adjacent endplate of the vertebra as distractor assembly 10 isinserted into the disc space. Surfaces 28 help retain the distractor 12in its inserted position during cage insertion. The roughened endplatesfacilitate bone growth by providing greater surface area for contactbetween the cage and the endplates, and also between the endplates andbone growth material placed within the cage.

Body portion 14 of each distractor has a lateral dimension D and aheight h1. It is preferred that body portion 14 be cylindrical in shape,and thus D is the same as h1 and constant between leading end 22 andtrailing end 24. However, other dimensions for D are also contemplatedherein such that body portion 14 has other cross-sections, such assquare, rectangular, ovoid, and elliptical, to name a few. In any event,height h1 corresponds to the height of the cage to be inserted in thedisc space. Lateral spacers 16 have a width w1 extending from bodyportion 14 at trailing end 24. The width of lateral spacer tapers toconverge with body portion 14 at leading end 22. It is also contemplatedin an alternate embodiment that lateral spacers 16 have a width atleading end 22 that is less than the w1. Central spacer 18 a has a widthw1 extending from body portion 14 a to guide surface 26 a. Width w1 ispreferably the same as the width of the lateral spacer 16 a at trailingend 22 a; however, a width w1 for central spacer 18 a that differs fromthe width of lateral spacer 16 a is also contemplated. Central spacer 18b has a width w2 extending from body portion 14 b to guide surface 26 b.Width w2 is preferably less than the width w1 of central spacer 18 a.Central spacers 18 and lateral spacers 16 have a height h2 that is lessthan height h1 of body portion 14. It is preferred that height h2 be 4to 6 millimeter less than height h1; however, other height differencesare also contemplated.

The overall lateral dimension (w1+D+w2) of distractor 12 b at itstrailing end 24 b preferably corresponds to the maximum cross-sectionaldimension of the trailing end of the cage to be inserted. The leadingend 22 b has a width corresponding to the maximum lateral width of theleading end of the cage to be inserted. Also, shaft assembly 40 hasshafts 40 a and 40 b, each having a width that corresponds to theoverall lateral dimension (w1+D+w2) of distractor 12 b. The cam 43 a hasa width sufficient to extend from shaft 41 a to contact shaft 41 b. Inan alternate embodiment, it is contemplated that shaft 40 a has a widththat is equal to the width (w1+D+w1) of trailing end 24 a of distractor12 a.

As shown in FIG. 1, longitudinal axes A1 and A2 of distractors 12 a and12 b are separated by a lateral separation distance S, and each areoffset a distance ½ S from central axis A of distractor assembly 10. Itis most preferred that this separation distance maximizes the positionof axes A1 and A2 in the disc space with respect the centerline of thespinal column. If it is desired or necessary to further increaseseparation distance S, a third distractor 12 c, such as that shown inFIG. 4, may be used in place of distractor 12 a. Distractor 12 c isidentical to distractor 12 a, except that central spacer 118 c has awidth w3 that is greater than width w1 of distractor 12 a. Use ofdistractor 12 c thus increases the separation distance S betweendistractor axes L3 and L2 by an amount corresponding to the increase ofwidth w3 over width w2. Alternatively, the surgeon may use a seconddistractor 12 a in place of distractor 12 b to increase the separationdistance S by an amount corresponding to the increase of width w1 overw2.

It is further contemplated herein that distractor assembly 10 ismodular, permitting interchangeability of various sized distractors 12with the shaft assembly used therewith in order to increase themedial-lateral spacing of the distractors and the disc distractionheight as needed. It is preferred that leading end 22 of the distractors12 incorporate the identical geometry of the fusion cage to be implantedin the distracted disc space. For example, various distractors 12 couldbe provided with height h1 ranging from 9 millimeters to 17 millimetersin increments of 1 millimeter. The surgeon selects distractors 12 havinga height h1 corresponding to the leading end of the cage to be insertedinto the disc space. It is also contemplated that distractor assembly 10can be used for final distraction of the disc space, i.e. as the lastdistractor inserted prior to insertion of the fusion cages or otherimplants into the disc space. Distraction of the disc space prior toinsertion of distractor assembly 10 may be accomplished using any knowndistractor instrumentation and technique.

Referring now to FIGS. 5(a) and 5(b), side elevational views ofalternate embodiments for distractor 12 are shown. In FIG. 5(a), thedistractor 12 has leading end 22 that is tapered to a height at leadingend 22 that is less than h1. The tapered leading end facilitatesinsertion of the distractor 12 into the disc space. In FIG. 5(b), theleading end 22 is rounded to form a “blunt nose” at leading end 22.Other shapes and configurations for leading end 22 are also contemplatedherein. In FIG. 6, lateral spacer 16 is shown with a rounded edge 17that extends from leading end 22 to trailing end 24. Rounded edge 17further facilitates and eases introduction of the distractor 12 into thedisc space.

Referring now to FIGS. 7-9, there is shown a further embodiment of thedistractor assembly according to the present invention. Distractorassembly 500 may be coupled to one or more obturators or shaftassemblies, as described above with respect to FIG. 3, for insertion ofdistractors 512 a and 512 b into a disc space between the endplates ofadjacent vertebra in order to restore the proper disc space height priorto insertion of a fusion cage or other implant therein. Distractorassembly 500 has a central axis A5 and includes a first distractor 512 aand a second distractor 512 b positioned adjacent first distractor 512a. Distractors 512 a and 512 b have central axes A6 and A7 separated bya lateral spacing S5.

Distractor 512 a includes a body portion 514 a around axis A6. Bodyportion 514 a extends between a leading end 522 a and a trailing end 524a. Extending medially from body portion 514 a is a central spacer ormedial portion 518 a. Medial portion 518 a defines a guide surface 526 aadjacent to and abutting body portion 514 b of second distractor 512 b.Formed with and extending proximally from trailing end 524 a of bodyportion 514 a is a connecting portion 530 a for connecting distractor512 a to a shaft. Connecting portion 530 a includes a guide arm 532 aextending proximally therefrom. Guide arm 532 a includes a mediallyfacing guide surface 534 a that is coplanar with and forms an extensionof guide surface 526 a of medial portion 518 a. As described furtherbelow, a shaft of an implant insertion instrument is positionableagainst guide surface 534 a during implant insertion to assist inmaintaining the proper positioning and spacing of the cage duringinsertion along guide surface 518 a. A notch 536 a is formed in theproximal end of arm 532 a.

Second distractor 512 b includes a body portion 514 b around a centralaxis A7. Body portion 514 b extends between a leading end 522 b and atrailing end 524 b. Distractor 512 b further includes a connectingportion 530 b extending proximally from trailing end 524 b of bodyportion 514 b for connecting distractor 512 b to a shaft. Connectingportion 530 b includes a proximally extending extension arm 531 b havinga protrusion 533 b extending medially therefrom towards first distractor512 a. Protrusion 533 b is positionable in notch 536 a to preventdistractors 512 a and 512 b from rotating relative to one another duringinsertion of distractor assembly 500 into the disc space. It iscontemplated that side of body portion 514 b abutting guide surface 526a is rounded convexly and that guide surface 526 a is rounded concavelyfit in close engagement with body portion 514 b.

Connecting portions 530 a and 53 lb each have a height h5 that isgreater than the height h4 of its connect body portion 514 a and 514 b,respectively. Further, distractor 512 a includes two anchoring members538 a extending from connecting portion 530 a towards leading end 522 aalong the top surface of body portion 514 a and two anchoring members538 a extending along the bottom surface of body portion 514 a.Similarly, distractor 512 b includes one anchoring member 538 bextending from connecting portion 531 b towards leading end 522 b alongthe top surface of body portion 514 b and one anchoring member 538 bextending along the bottom surface of body portion 514 b. It will beunderstood that more or fewer anchoring members 538 a, 538 b can beprovided, and that anchoring members 538 a, 538 b may be provided onlyalong the top surface or bottom surface of the distractor body portions514 a and 514 b. Anchoring members 538 a, 538 b are preferably wedgeshaped so as to engage into the cortical bone of the vertebral endplateas the distractor assembly 500 is driven into the disc space so as toresist lateral migration of distractor assembly 500 and distractors 512a, 512 b during the surgical procedure.

Like components of first and second distractors 512 a and 512 b may berecited herein collectively by referring to, for example, body portion514. A shaft assembly, such as that described above with respect to FIG.3, may be provided with distractor assembly 500. As shown in FIG. 9, theproximal or trailing end wall of distractor 512 a includes threadedopening 520 a formed in connecting portion 530 a and the proximal ortrailing end wall of distractor 512 b includes threaded opening 520 bformed in connecting portion 531 b. Threaded portion 42 a of shaft 41 acan be threaded into opening 520 a and threaded portion 42 b of shaft 41b can be threaded into opening 520 b.

To insert the distractors 512 a and 512 b in the disc space, eachdistractor 512 a and 512 b is connected with the corresponding threadedportion 42 a and 42 b of shafts 41 a and 41 b, respectively. Theproximal ends of the shafts 41 a and 41 b are secured by an impactingcap that holds the shafts together and distributes a driving forcebetween distractor 512 a and distractor 512 b to simultaneouslydistractors 512 a and 512 b into the disc space.

In one preferred embodiment, each distractor 512 has a knurled orroughened surface in the form of teeth 528 on the top and on the bottomof body portion 514 adjacent the vertebral endplates. For ease ofinsertion, it is preferred that the surface of body portion 514 besubstantially smooth and rounded adjacent leading end 522 and that teeth528 not extend to leading end 522. It is also contemplated that leadingend 522 of body portion 514 has a height h3 corresponding to the rootdiameter height of the leading end of the cage to be inserted. Teeth 528can be made by any combination or pattern of indentations or recessesand projections formed on the top and bottom surfaces of body 514. Eachtooth 528 can also be configured to scrape or remove a portion of bonematerial from the adjacent endplate of the vertebra as distractorassembly 500 is inserted into the disc space. Teeth 528 help retaindistractor 512 in its inserted position during cage insertion. Theroughened vertebral endplates facilitate bone growth by providinggreater surface area for contact between the cage and the endplates, andalso between the endplates and bone growth material placed within thecage.

Medial portion 518 a has a width w1 extending from the medial most edgeof body portion 514 a to the portion of guide surface 526 a closest tobody portion 514 a. Width w3 is sized to achieve the desired lateralspacing between the distractor 512 a and 512 b and subsequently thefusion cages that will be inserted into the disc space locationsinitially occupied by these distractors. For example, in one specificembodiment, medial portion 518 a provides a spacing w3 of 2 millimeters.In another specific embodiment, medial portion 518 a provides a spacingof 4 millimeters. Other spacing dimensions are also contemplated. Medialportion 518 a has a height h4 that is less than height h3 of bodyportion 514 a. It is preferred that height h4 be 4 to 6 millimeter lessthan height h3; however, other height differences are also contemplated.Medially extending portion 518 a has a length extending from thetrailing end of body portion 514 a to a position proximate leading end522 a, and in the illustrated embodiment has a length aboutthree-fourths the length of body portion 514 a.

As shown in FIG. 8, longitudinal axes A6 and A7 of distractors 512 a and512 b are separated by a lateral separation distance S5, and each areoffset a distance ½ S5 from central axis AS of distractor assembly 500.It is preferred that this separation distance maximizes the separationof axes A6 and A7 in the disc space with respect the centerline of thespinal column. Guide surface 526 a is offset laterally from central axistowards second distractor 512 b.

It is further contemplated herein that distractor assembly 500 ismodular, permitting interchangeability of various sized distractors 512with each other and with the shaft assembly used therewith in order toincrease or decrease the medial-lateral spacing of the distractors andthe disc distraction height as needed. It is preferred that leading end522 of distractors 512 incorporate the identical geometry of the fusioncage to be implanted in the distracted disc space. For example, variousdistractors 512 could be provided with height h3 ranging from 9millimeters to 17 millimeters in increments of 1 millimeter. The surgeonselects distractors 512 having a height h3 corresponding to the leadingend of the cage to be inserted into the disc space. It is alsocontemplated that distractor assembly 500 can be used for finaldistraction of the disc space, i.e. as the last distractor insertedprior to insertion of the fusion cages or other implants into the discspace. If necessary, distraction of the disc space prior to insertion ofdistractor assembly 500 may be accomplished using any known distractorinstrumentation and technique.

After insertion of distractor assembly 500 in the disc space withleading ends 522 a and 522 b at the proper depth distally in the discspace, and the distractor axes A6 and A7 at the desired lateral spacingS5, distractor 512 b may be removed from the distracted disc space withdistractor 512 a remaining in its inserted position. The distractionchannel formed in the disc space by removed distractor 512 b acts as aguide for a first fusion cage to follow during insertion. Distractor 512a also acts as a guide for insertion of the first fusion cage into theportion of the disc space occupied by distractor 512 b. The distractionchannel and guide surface 526 a of medial portion 518 a maintain thelateral positioning of the first fusion cage with respect to axis A anddistractor 512 a, and also resists medial migration of the first cage inthe disc space during its insertion. After the first cage is inserted,distractor 512 a is removed and the first cage guides insertion andresists medial migration of the second cage as it is inserted into thedisc space. It is preferred that the cages be threaded to resist backoutfrom the disc space, and are also preferably tapered to restore spinallordosis.

Referring to FIG. 10, there is illustrated an example of a segment ofthe spinal column to which the present invention has application. Spinalcolumn segment 50 includes adjacent vertebra V1 and V2 and disc space DStherebetween around a central axis 68. Annulus 64 surrounds disc spaceDS and extends between vertebra V1 and V2. Each vertebra V1, V2 includesa bony endplate 52, 54 and an outer peripheral ring 60, 62, surroundinga softer inner portion 56, 58, respectively. Endplates 52, 54 each havea thickness t and typically include concave areas 53 and 55,respectively. Endplates 52, 54 and peripheral rings 60, 62 are made fromhard bony material, and provide the lateral stability and loaddistributing capabilities of a healthy spinal segment 50.

Distractors 12 a, 12 b and 512 a, 512 b may each form a channel at adesired lateral location in an unreamed disc space for the cage tofollow during insertion. Distractor assembly 10 and 500 are particularlyuseful to provide and maintain the desired lateral positioning of thecages with respect to central axis 68 within an unreamed the disc spaceDS so each cage may be implanted in the desired position whilepreserving the thickness t of bony endplates 52, 54. Distractor assembly10 and 500 also increases the lateral spacing between fusion cages withrespect to axis 68 to increase load distribution to peripheral rings 60,62 and increase the lateral stability of the spinal column segment. Thisincrease in lateral spacing is illustrated in FIGS. 11 and 12. FIG. 11shows a partial cross-sectional view of two prior art interbody fusioncages 310 and 320 placed in a disc space D4 disposed between adjacentvertebral bodies V3 and V4. As used herein, the term cage is intended toencompass any prior art fusion device or implant. Disc space D4 has aheight H4. The fusion cages 310 and 320 have a height H5. The height H5of the fusion cages is greater, typically by 4 to 6 mm, than the heightH4 of the disc space. Thus, openings 312, 314, 322, and 324 must beformed in the vertebra to accommodate the extra height of the implants.These openings through the stronger bone of the endplates tend to weakenthe bony area adjacent the implants, increasing the risk of implantsubsidence. Further, the increased size of the implants requires them tobe placed close together to avoid extending beyond the lateral edges ofthe disc space. The spacing between the centerline of cages 310 and 320is distance S4.

Referring to FIG. 12, the present invention provides improved interbodyfusion devices. Disc space D6 is disposed between vertebrae V5 and V6,and has a height H6. For purposes of illustration, height H6 issubstantially identical to height H4 of FIG. 24. Thus, cages of thepresent invention, represented by cages 330 and 340, are positioned indisc space D6 with the endplates of vertebrae V5 and V6 substantiallyintact and supported by cages 330 and 340. The intact cortical bonedecreases the risk of subsidence of cages 330 and 340 into the vertebraeV5 and V6. Cages 330 and 340 have substantially the same lateral spacingfrom the sides of the disc space as prior art cages 310 and 320 of FIG.11. However, the centerline spacing S6 is substantially greater thancenterline spacing S4. Thus, a void 350 is created that may be filledwith a greater volume of bone in-growth material than possible with theprior art cages of FIG. 11. Further, the endplate contacts 332, 334,342, and 344 are spaced farther apart thereby increasing the stabilityof the construct and subsequent fusion.

In accordance with this aspect of the invention, there is provided afusion cage 200 as shown in FIGS. 13-17. Fusion cage 200 is particularlysuited for insertion into an unreamed disc space in either the lumbar orcervical regions of the spine. Cage 200 eliminates the need for achannel discectomy and/or reaming of the disc space prior to insertionof the cage 200. Fusion cage 200 does not require a cannula, guide tube,or laparoscopic instrument for insertion into the disc space, althoughthe use of such instruments and techniques associated therewith are notprecluded by cage 200. It is also contemplated that the fusion cage 200may be inserted using the techniques and instruments described herein.However, other known techniques and instruments may also be used toinsert these cages.

Cage 200 includes body 202 extending between leading end 204 andtrailing end 206. A number of threads or single thread 208 extend aroundbody 202 between leading end 204 and trailing end 206 transverse to acentral axis C. Body 202 defines a number of side apertures 212 insidewalls 224 that communicate with a hollow interior 214. Cage 200 hasa top bearing surface 219 that is adjacent the superior vertebralendplate when cage 200 is inserted. At top bearing surface 219 body 202defines a number of top apertures 220. In a preferred embodiment, thereare provided four top apertures 220 that are substantially the same sizeand symmetrically positioned about axis C. A bottom bearing surface 221includes a number of bottom apertures 222 corresponding in size, shapeand location to top apertures 220. It is desirable that hollow interior214 be filled with BMP material, bone graft, chips or other bone growthcompound to effect fusion between the vertebrae.

An end cap or end nose 216 is provided at leading end 204 and formedwith body 202. End nose 216 has a recess 217 formed therein along acenter axis of cage 200. End nose 216 further includes an opening 218formed therethrough at the center of recess 217 that communicates withinterior 214 to provide a path for blood flow through leading end 204 ofcage 200, further increasing the porosity of cage 200 for fusion. Recess217 allows opening 218 to be offset proximally or towards trailing end206 in relation to the distal-most end of leading end 204. This protectsthe tissue in the disc space from contact with sharp or abrupt edgesthat might be formed around hole 218. End nose 216 preferably has arounded configuration between the top and bottom bearing surfaces thatmatches the profile of the distractor used to distract the disc space.This allows end nose 216 to also distract the disc space, if necessary,as cage 200 is threaded into the disc space. Preferably, end nose 216 isunthreaded and has a length along axis C that corresponds to about 15%of the overall length of cage 200 between leading end 204 and trailingend 206.

For ease of insertion and maintenance of position in the disc space, itis contemplated that body 202 includes one or more threads 208 along atleast a portion of the length of body 202 that are self-tapping.Preferably, threads 208 are spaced sufficiently such that body 202 cancontact the vertebral endplates between adjacent ones of the threads. Inone specific embodiment, thread 208 has a pitch of 3 so that cage 200advances 3 mm into the disc upon a complete revolution of cage 200 aboutaxis C. It is also contemplated that the threads gradually increase indepth from d1 to d2 as threads 208 run from leading end 204 to trailingend 206. In one specific embodiment, it is contemplated that depth d1will correspond to about 1.0 mm and depth d2 will correspond to about1.5 mm.

The form of threads 208 facilitate the cutting of threads 208 into thecortical bone of the intact vertebral endplates in the unreamed discspace. As shown in further detail in FIG. 13a, thread 208 includes asharp crest 230 that may be slightly truncated, a concave trailing wall232 extending from crest 230 to body 202, and a convex leading wall 234extending from crest 230 to body 202. Sharp crest 230 cuts into thecortical bone and advance cage 200 as it is threaded into the discspace. Such a configuration prevents the adjacent vertebral bodies fromriding up on threads 208, thereby preventing the disc space height fromincreasing substantially above body 202 of cage 200 as it is threadedinto position. The cortical bony endplates are thus maintained insubstantial contact with body 202 between threads 208, and threads 208are embedded into the cortical bone of the endplates. Undesirablesubsidence of the vertebrae along threads 208 is avoided, and threads208 secure cage 200 to the cortical bone of the vertebral endplatesrather than the cancellous bone. Subsidence is also resisted by body 202of cage 200 body 202 supports the hard cortical bone of the adjacentvertebral endplates. Resistance to expulsion of cage 200 from the discspace is also increased since thread 208 achieves greater purchase intothe cortical bone.

As cage 200 is threaded into the disc space, the material harvested bythread 208 is scaved and deposited through openings 220 and 222 and intohollow interior 214. This depositing of material is enhanced by theconfiguration of thread 208. As shown in FIG. 13a, crest 230 has avertically oriented midline M1 perpendicular to central axis C of cage200. The base of thread 208 has a second midline M2 perpendicular tocentral axis C. Midline M2 is distally offset from midline MI towardsleading end 204. This swept back profile of thread 208 and concavetrailing wall 232 assist in depositing bony material into openings 220and 222 as cage 200 is threaded into the disc space.

Body 202 is tapered along its length from height H1 at leading end 204to height H2 at trailing end 206 to define an angle that restores thenatural curvature of the spine when inserted into the disc space. Thetapered body 202 further distracts the proximal portion of the discspace in accordance with the increase in height associated with thetaper of body 202 as cage 200 is threaded into the disc space. As cage200 is inserted, threads 208 achieve purchase into the intact corticalbone of the adjacent vertebral endplates. The increase in thread depthfrom leading end 204 to trailing end 206 provides greater purchase ofthe portion of thread 208 adjacent trailing end 206 into the bonyendplate, and, along with the swept back profile of thread 208, reducesthe risk of cage 200 backing out or un-threading from the disc space.Thus, maintenance of cervical and lumbar lordosis is improved and therisk of subsidence associated with implants inserted into a reamed discspace is eliminated.

Other advantages realized by cage 200 are associated with its relativelysmaller size as compared to cages inserted in reamed openings. Since theendplates of the vertebrae are not reamed, the overall heights H1 and H2of cage 200 are less than that required for a cage inserted into areamed disc space. Typically, heights H1 and H2 will be about 4 to 6millimeters less than the corresponding heights of a cage for a reameddisc space. Also, the concave sidewalls 224 of cage 200 reduce thelateral dimension of the inserted cage 200. Thus, if two cages 200 arebilaterally inserted into the disc space, greater separation distancecan be realized than that for cages bilaterally inserted in a reameddisc space or for cages having a lateral dimension that approximates theheight of the cage. This allows the fusion cage 200 to be positionedcloser to the peripheral bony ring of the vertebral body, resulting inincreased lateral stability and more load distributed at the strongestportions of the adjacent vertebrae. The cage 200 also enables bilateralfusion cage placement in a smaller sized disc space since less lateralwidth in the disc space is required to accommodate the fusion cages 200.

Another embodiment of a fusion cage is shown in FIGS. 18-22. Asdiscussed above with respect to fusion cage 200, fusion cage 400 is alsoparticularly suited for insertion into an unreamed disc space in eitherthe lumbar or cervical regions of the spine. It is also contemplatedthat fusion cage 400 may be inserted using the techniques andinstruments described herein, among others.

Cage 400 includes body 402 extending between leading end 404 andtrailing end 406. One or more threads 408 extend around body 402 betweenleading end 404 and trailing end 406 transverse to a central axis C.Body 402 defines a number of side apertures 412 in sidewalls 424 thatcommunicate with a hollow interior 414. An end plate 416 is provided atleading end 404. End plate 416 defines a number of openings 418therethrough communicating with interior 414 to further increase theporosity of cage 400 for fusion. A tapered portion 417 extends fromleading end to body 402, and preferably includes a shape that correspondto that of the previously inserted distractor to facilitate insertion ofthe cage 400 into the disc space. In one specific embodiment, taperedportion 417 is unthreaded and has a length of about 2.5 millimeters.

As shown in FIG. 22, cage 400 has a top surface 419 that is adjacent thesuperior vertebral endplate when cage 400 is inserted. At top surface419 body 402 defines a number of top apertures 420. In a preferredembodiment, there are provided four top apertures 420 that aresubstantially the same size and symmetrically positioned about axis C. Abottom surface 421 includes a number of bottom apertures 422corresponding in size, shape and location to top apertures 420. It isdesirable that hollow interior 414 be filled with BMP material, bonegraft, chips or other bone growth compound to effect fusion between thevertebrae.

For ease of insertion and maintenance in the disc space, it iscontemplated that body 402 includes threads 408 along at least a portionof the length of body 402 that are self-tapping. It is also contemplatedthat threads 408 gradually increase in depth from d1 to d2 as threads408 run from leading end 404 to trailing end 406. In this embodiment ofcage 400, threads 408 have a sloped crest 430 extending between trailingwall 432 and a leading-wall 434, forming a substantially rectangularthread profile. As cage 400 is threaded into the disc space, thematerial harvested by the threads 408 is deposited through openings 420and 422 and into hollow interior 414. Body 402 is tapered along itslength from height H1 at leading end 404 to height H2 at trailing end406 to define an angle that restores the lordotic angle of the spinewhen inserted into the disc space. The tapered body 402 furtherdistracts the disc space in accordance with the increase in heightassociated with the taper of body 402 as cage 400 is threaded into thedisc space. As cage 400 is inserted, threads 408 achieve purchase intothe intact bony endplates of the adjacent vertebrae. The increase inthread depth from leading end 404 to trailing end 406 allow greaterpurchase of the threads adjacent trailing end 406 into the bonyendplate, thus reducing the risk of cage 400 backing out or un-threadingfrom the disc space. Thus, maintenance of cervical and lumbar lordosisis improved and the risk of subsidence associated with implants insertedinto a reamed disc space is eliminated.

Other advantages realized by cage 400 are associated with its relativelysmaller size as compared to cages inserted in reamed openings. Since theendplates of the vertebrae are not reamed, the overall heights H1 and H2of cage 400 are less than that required for a cage inserted into areamed disc space. Typically, heights H1 and H2 will be about 4 to 6millimeters less than the corresponding heights of a cage for a reameddisc space. Also, the sidewalls 424 of cage 400 each include a cutout426 that reduces the lateral dimension of the inserted cage 400. Thus,if two cages 400 are bilaterally inserted into the disc space, greaterseparation distance can be realized than that for cages bilaterallyinserted in a reamed disc space or for cages having a lateral dimensionthat approximates the height of the cage. This allows the fusion cage400 to be positioned closer to the peripheral bony ring of the vertebralbody, resulting in increased lateral stability and more load distributedat the strongest portions of the adjacent vertebrae. The cage 400 alsoenables bi-lateral fusion cage placement in a smaller sized disc spacesince less lateral width in the disc space is required to accommodatethe fusion cages 400.

In one embodiment, cage 400 includes tool guides 428 at trailing end406, as shown in FIG. 21. Tool guides 428 are configured to receive andguide the movement of the shaft of an instrument inserted into interior414 of the cage 400. Cage 200 described above could similarly beprovided with such tool guides.

Referring now to FIGS. 23 and 24, there is illustrated a spacer 600engagable to a spacer insertion shaft 602. Spacer 600 is connectable toa fusion cage, such as the illustrated fusion cage 200, in order toguide and maintain lateral spacing of a second fusion cage (not shown)during insertion of the second cage into the disc space. Spacer 600includes a cage connector 604 that extends into the hollow interior ofcage 200. Preferably, connector 604 has a cross-section shape that fitswithin the trailing end wall opening of cage 200 such that spacer 600will be non-rotatably secured to cage 200. Extending proximally fromconnector 604 is a proximal end portion 608 that includes a proximal endwall with a threaded opening (not shown) to engage spacer 600 to shaft602. Extending from the medial side of proximal end portion 608 is aspacing member 606 that is separated from connector 604 by gap 610. Gap610 is sized such that a medial sidewall of cage 200 can fit thereinwhen spacer 600 is connected to the inserted cage 200 with connector 604in the interior of the cage and spacing member 606 positioned along thesidewall of cage 200. Spacing member 606 has a thickness t thatcorresponds to the desired spacing between the inserted fusion cages.Spacing member 606 has a concave guide surface 612 along its medial sideto guide threaded insertion of the second cage. Spacing member 606 andguide surface 612 preferably extend to the trailing end wall of proximalend portion 608 and outside the disc space to support and guide theshaft of the implant insertion device. Spacing member 606 also includesa convex lateral sidewall that conforms with the concave sidewall ofcage 200 to provide a secure and snug fit between cage 200 and spacerdevice 600. After the second cage is inserted into the disc space,spacer 600 is removed from first cage 200.

Referring now to FIGS. 25(a) to 25(h), one preferred method according tothe present invention will be described with respect to cage 200,distractor assembly 500, and spacer device 600. It will be understood,however, that the other embodiments of cages, spacers, and instrumentsdescribed herein also have application with the described method. InFIG. 25(a) there is illustrated distractor assembly 500 prepared forinsertion in the disc space. The disc space is accessed using any knownsurgical technique, including those which anteriorly approach the discspace through an open incision, through one or more tubes or through adouble barrel tube, and through laparoscopic instruments that provide asealed working channel. The surgical site is prepared for entry ofdistractor assembly 500, and distractors 512 a and 512 b are assembledand positioned adjacent the disc space. Although the distractors can beinserted individually, it is preferred that the proximal ends of thedistractor shafts 41 a, 41 b are coupled with an impactor cap anddistractors 512 a, 512 b driven into the disc space simultaneously.

In FIG. 25(b) distractor assembly 500 has been inserted and seconddistractor 512 b withdrawn. Fusion cage 200 is coupled to inserter shaft700. Guide arm 532 a guides insertion of cage 200 to the disc space, andmedial portion 518 a guides cage 200 as it is threaded into the discspace with inserter shaft 700 guided by guide arm 532 a until cage 200is threaded to the desired position in the disc space, as shown in FIG.25(c).

In FIG. 25(d) first distractor 512 a is removed and spacer 600 issecured to shaft 602. Connector 604 of spacer 600 is secured to cage 200with spacing member 606 positioned along the medial side of cage 200 asshown in FIG. 25(e). In FIG. 25(f) shaft 602 is removed and a secondcage 200′ is connected to implant inserter shaft 700. Spacing member 606guides insertion of cage 200′ as it is threaded into the disc space asshown in FIG. 25(g). As shown in FIG. 25(h), spacer 600 is removed fromcage 200 and cages 200, 200′ are inserted into the disc space and thedesired lateral spacing between cages 200, 200′ has been achieved.

In accordance with another aspect of the invention, there is provided inFIGS. 26-27 a fusion cage 150 that is insertable into an unreamed discspace distracted by the distractor assembly of the present invention.While it is contemplated that two fusion cages 150 are bilaterallyinserted into the disc space, it is also contemplated that a singlefusion cage 150 or more than two fusion cages 150 may be inserted intothe unreamed disc space.

Fusion cage 150 includes body 152 extending between trailing end 154 andleading end 156. Body 152 includes a plurality of threads 158 extendingat least partially therearound for partially cutting into the thicknessof the vertebral endplates. Threads 158 preferably have a depth thatincreases from p 1 at leading end 156 to p2 at trailing end 154.Providing deeper threads towards trailing end 154 increases back-outresistance of cage 150 in the disc space. A cage 150 having threads 158with a constant depth is also contemplated. Top surface 170 includesupper aperture 162 and bottom surface 172 includes a lower aperture (notshown) communicating with hollow interior 168. A plurality of openings166 communicating with hollow interior 168 is provided between topsurface 170 and bottom surface 172 through sidewalls 153 to increase theporosity of cage 150 for fusion. Trailing end 154 of body 152 mayinclude notch 174. Notch 174 is configured to receive a de-rotation bar(not shown) therethrough. The de-rotation bar is placed in notch 174 ofa first fusion cage 150 and extends to a notch of a second fusion cage(not shown) bi-laterally inserted with the first fusion cage 150. Thede-rotation bar prevents rotation of the fusion cages in the disc space.

In FIGS. 28-29, there is shown another embodiment spacer device 180.Spacer device 180 includes a spacing member 182 having a thickness t1and a cage connector 184 extending from body portion 182. Spacing member182 and connector 184 define a recess 186 therebetween. As shown in FIG.30, after insertion of cage 150 into the disc space, the spacer 180 maybe connected to the medial side of cage 150 and used as a guide forbi-lateral placement of a second cage in the disc space. The recess 186receives a portion of the wall of body 152 at trailing end 154 andconnects the spacer 180 with cage 150 along medial sidewall 153. Spacingmember 182 extends from trailing end 154 along body 152 forsubstantially it entire length. The thickness t1 of spacing member 182is determined by the desired separation distance between the first andsecond cages.

Another advantage realized by cage 150 is that the portion of theendplate communicating interior 168 through apertures 162 may be removedor reduced to bleeding in order to obtain the advantages associated withimplants inserted into a reamed disc space. As described below, acutting instrument with a curette or burr is inserted into the interiorof the inserted cage 100 to remove bony material from the vertebralendplates through the apertures 162 in the top surface 170 and bottomsurface 172. The remaining portions of the endplates remain intact andprovide a strong bearing surface in contact with body 152 and threads158. Bone growth material may then be placed in hollow interior 168, andfusion between the vertebrae is attained in a manner realized by afusion cage inserted into a reamed disc space while retaining theincreased stability and load distribution capabilities associated withinserting the fusion cage 150 in an unreamed disc space.

An alternate embodiment cage 150′ is shown in FIG. 31. Cage 150′ issimilar to cage 150, but has sidewalls 153′ that curve inwardly betweentop surface 170′ and bottom surface 172′. Spacer device 180 would beshaped accordingly to conform to sidewall 153′ to allow connectionbetween spacer device 180 and cage 150′. Thickness t1 of body portion182 is thus increased due to the concavity of sidewall 153′. A cage 150′having concave sidewalls 153′ allows the distraction height and threaddepth of the cage to be increased without a corresponding increase inthe lateral portion of the disc space occupied by the cage.

There is provided a further embodiment of a fusion in FIGS. 32-34.Fusion cage 100 is designed for insertion into an unreamed disc space ineither the lumbar or cervical regions of the spine. Insertion of cage100 in an unreamed disc space eliminates the need for a channeldiscectomy and/or reaming of the disc space prior to insertion of thecage 100. Fusion cage 100 does not require a cannula, guide tube, orlaparoscopic instrument for insertion into the disc space, although theuse of such instruments and techniques associated therewith are notprecluded by cage 100. It is also contemplated that fusion cage 100 maybe inserted using the techniques and instruments described herein, amongothers.

Cage 100 includes body 102 extending between leading end 104 andtrailing end 106. A number of struts 108 a, 108 b and 108 c arepositioned between ends 104 and 106, and extend transverse to a centralaxis L of cage 100 around body 102. A longitudinal strut 110 extendsalong axis L between ends 104 and 106 along body 102. Although only onestrut 110 is shown in FIG. 13, it is contemplated that additional struts110 are positioned about body 102 along axis L. Struts 108 and 110strengthen and support body 102. Body 102 defines a number of largeopenings 112 that communicate with a hollow interior 114. A blunt nose116 is provided at leading end 104. Blunt nose 116 pushes disc materialfrom leading end 104 and around body 102 as cage 100 is inserted. A cap118 may be placed on trailing end 106 to close hollow interior 114. Itis desirable that hollow interior 114 be filled with BMP material, bonegraft, chips or other bone growth compound to effect fusion between thevertebrae before placement of end cap 118. Cap 118 may be threaded tomate with corresponding threads formed at end 106 in the interior 114 ofbody 102, or cap 118 may be press fit into hollow interior 114 at end106.

For ease of insertion, it is contemplated that body 102 includes one ormore threads 122 therearound along at least a portion of the length ofbody 102. Preferably, the material harvested by the threads 122 of body102 is deposited through openings 112 and into hollow interior 114. Body102 is tapered along its length from height H1 at trailing end 106 toheight H2 at leading end 104 to define an angle that corresponds to thelordotic angle of the spine when inserted into the disc space. The bluntnose 116 and strengthened tapered body 102 distract the disc space ascage 100 is inserted therein, thus completely or assisting indistracting the disc space. As cage 100 is inserted, the threads 122achieve purchase into the intact bony endplates of the adjacentvertebrae. Thus, maintenance of cervical and lumbar lordosis is improvedby eliminating the risk of subsidence associated with an implantinserted into a reamed disc space. The cage 100 is also inserted in aposition that accommodates the anatomy of the disc space since the cage100 is self-directed as it is inserted therein.

Other advantages realized by cage 100 are associated with its relativelysmaller size as compared to cages inserted in reamed openings. Since theendplates of the vertebrae are not reamed, the overall heights H1 and H2of cage 100 are less than that required for a cage inserted into areamed disc space. Typically, heights H1 and H2 will be about 4 to 6millimeters less than the corresponding heights of a cage for a reameddisc space. Thus, if two cages 100 are to be inserted bilaterally intothe disc space, greater separation distance can be realized than thatfor cages bilaterally inserted in a reamed disc space. This allows thefusion cage 100 to be positioned closer to the peripheral bony ring ofthe vertebral body, resulting in increased lateral stability and moreload distributed at the strongest portions of the adjacent vertebrae.

In accordance with a further aspect of the invention, variousinstruments are provided for disc space and vertebral endplatepreparation, as shown in FIGS. 35-37. In FIG. 35, cutting instrument 250includes a shaft 252 extending from handle 254 to a distal portion 256.Preferably, distal portion 256 is deflected at an angle R with respectto shaft 252. A curette or blade 258 is positioned on the end of distalportion 256. Blade 258 is sharpened and configured to remove bonematerial from the vertebral endplates upon application of a forceapplied through handle 254 by the surgeon. Shaft 252 and distal portion256 extend into a fusion cage inserted in the disc space, such as thecage 100 or 150 described above, or the cage 200 described below. Blade258 extends from the interior of the cage through openings in the fusioncage, such as openings 162 of cage 150; the openings 112 of cage 100; orthe openings 220 of cage 200. The blade 258 contacts the portion of thevertebral endplate communicating with the openings in the cage.

Cutting instrument 250 provides an instrument that allows the surgeon toremove bony material from the endplates of the vertebral bodies afterthe fusion cage is inserted into the disc space. Subsidence and settlingof the implant into the adjacent vertebrae is avoided since the body ofthe fusion cage is fully supported by the remaining bony material of theendplates. As realized in procedures that utilize a reamed disc space,superior fusion may be achieved by removing the bony endplates topromote bone growth between the vertebral bodies through the fusionimplant. However, the instruments of the present invention remove aportion of the bony endplates only at the areas where the endplatescommunicate with the opening in the cage.

FIG. 36 is an alternate embodiment cutting instrument 270 having a shaft272 extending from handle 274. A distal portion 276 of shaft 272 isdeflected upward and an angle R with respect to the shaft, as describedabove with respect to curette 250. Cutting instrument 270 is identicalto cutting instrument 250, except that distal portion 276 includes aforked portion 280 having a pair of blades 278. Blades 278 arepreferably laterally spaced from one another. Cutting instrument 270 hasparticular application to a fusion cage having a pair of holescommunicating with the vertebral endplate that are laterally adjacentone another. See, for example, apertures 112 of fusion cage 100 andapertures 220 of fusion cage 200 (described below.) Cutting instrument270 allows the surgeon to simultaneously and uniformly remove bonymaterial through adjacent holes in the fusion cage.

The present invention also contemplates that cutting instruments 250 and270 include a mechanical burr or reciprocating blade in place of blades258 and 278. The instruments 250 and 270 include circuitry and a motorfor connection to a power source that drives the mechanical burr toremove bony material from the endplates.

One preferred technique using these instruments will be describe withrespect to cage 400, It being understood that the technique couldsimilarly be used with the other cages described herein or other cagesknown in the art. Cutting instruments 250 or 270 is inserted into cage400 and used to prepare the vertebral endplates, as discussed above,through top apertures 420 and bottom apertures 422. The guides 428receive the shaft 252 or 272 to facilitate endplate preparation bymaintaining the positioning of the cutting instruments 250, 270 as it ismanipulated within the fusion cage. The guide also allows the shaft tobear securely therein so the force applied by the surgeon to thevertebral endplates with blades 258, 278 may be increased.

FIG. 37 is a partial elevational view of another instrument of thepresent invention. Channel starter 290 is used to prepare a channelthrough the anterior lip of the vertebral endplates at the disc spaceportion adjacent trailing end 24 when distractor 12 is positionedtherein. Channel starter 290 includes an outer shaft 291 and an innershaft 292. A cutting blade 295 is positioned between outer shaft 291 andinner shaft 292. Inner shaft 292 has an end portion 293 that ispreferably threaded and configured to be received within threadedopening 20 of distractor 12. Inner shaft 294 acts as a guide for theouter shaft 291 and cutting blade 295. A stop 294 is positioned on innershaft 292 and contacts trailing end 24 to limit insertion of cuttingblade 295 to a predetermined depth within anterior portion of the discspace. Cutting blade 295 removes a portion of the endplate thickness atthe anterior lip of the vertebral bodies, thus forming a startingchannel in the disc space coextensive with the distractor channel formedin the disc space by body portion 14 of distractor 12. The startingchannel facilitates insertion of the fusion cage in the disc space byproviding a directional guide for insertion of the leading end of thecage. The starting channel also maintains alignment of the body of thefusion cage throughout its insertion.

It is contemplated that the fusion cages of the present invention may beinserted using the techniques and instruments described herein. However,other known techniques and instruments 36, may also be used to insertthese cages. The cages of the present invention may be inserted as asingle cage in the disc space. The cages of the present invention canalso be inserted bi-laterally such that the bi-laterally inserted cageshave no spacing therebetween. Further, the cages can be insertedbi-laterally and provided with concave sidewalls so that thebi-laterally inserted cages overlap and provide a negative spacing.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. An apparatus insertable in a disc space betweenadjacent vertebrae, comprising: a first fusion cage positionable at afirst location in the disc space, said first fusion cage having a bodyextending between a leading end and a trailing end and defining a hollowinterior opening at said trailing end; and a spacer having a connectorpositionable in said trailing end opening of said first cage to securesaid spacer to said first fusion cage, said spacer further including aspacing member spaced from said connector and positionable outside ofsaid body when said spacer is secured to said first fusion cage, saidspacing member having a guide surface opposite said first fusion cage,wherein said guide surface guides the insertion of a second fusion cageat a second location in the disc space that is spaced from said firstfusion cage.
 2. The apparatus of claim 1, wherein said body of saidfirst fusion cage includes threads extending outwardly therefrom toengage the cortical bone of the adjacent vertebral endplate.
 3. Theapparatus of claim 1, wherein said connector has a cross-sectional shapethat is received in form fitting engagement with a trailing end wallopening of said cage.
 4. The apparatus of claim 1, further comprisingbone growth material in said hollow interior of said first fusion cage.5. The apparatus of claim 1, wherein said body includes an upper bearingsurface and a lower bearing surface separated by a spacing distance,said spacing distance of said body being selected so that said bodycontacts the cortical bone of the upper vertebral endplate and thecortical bone of the lower vertebral endplate when the device isinserted in an unreamed disc space.
 6. The apparatus of claim 5,wherein: said body includes concave sidewalls extending between saidupper and lower bearing surfaces; and said spacing member includes aconvex lateral surface opposite said guide surface positionable in oneof said concave sidewalls.
 7. The apparatus of claim 1, wherein saidbody includes an end nose at said leading end.
 8. The apparatus of claim7, wherein said end nose includes a rounded configuration and a recessedhole formed through said nose communicating with said hollow interior.9. The apparatus of claim 1, wherein said body of said first fusion cageincludes one or more threads engageable to cortical bone of the adjacentvertebrae with upper and lower bearing surfaces of said body contactingan adjacent vertebral endplate of the adjacent vertebrae.
 10. Theapparatus of claim 9, wherein said body defines a number of openings insaid upper bearing surface and a number of openings in said lowerbearing surface providing communication between said hollow interior andthe adjacent vertebrae.
 11. The apparatus of claim 10, wherein said oneor more threads are configured to deposit bone from the adjacentvertebral endplate through said openings into said hollow interior assaid first fusion cage is threaded into the disc space.
 12. Theapparatus of claim 9, wherein said one or more threads increase in depthfrom said leading end to said trailing end.
 13. The apparatus of claim12, wherein said one or more threads include: a crest; a leading faceextending from said body to said crest; a trailing face extending fromsaid body to said crest, wherein at a midpoint between said leading faceand said trailing face at its junction with said body, said crest ispositioned more proximate said trailing end than said midpoint.
 14. Theapparatus of claim 13, wherein said leading face is convex and saidtrailing face is concave.
 15. The apparatus of claim 14, wherein saidthreads have a depth of 1 mm at said leading end and a depth of 1.5 mmat said trailing end.
 16. A method for inserting one or more fusioncages in the disc space between an upper vertebra and a lower vertebrae,comprising: providing a fusion cage including a threaded body having ahollow interior extending between a trailing end and a leading end, thebody further including an upper bearing surface and a lower bearingsurface separated by a height, the height being substantially the sameas the desired spacing between the intact endplates of the uppervertebra and lower vertebra, respectively; accessing a disc space;distracting the disc space; and inserting the fusion cage in the discspace with endplates of the upper and lower vertebra substantiallyintact and in contact with the upper and lower bearing surfaces,respectively, of the fusion cage.
 17. The method of claim 16, whereinsaid height is tapered to restore lordosis.
 18. The method of claim 16,wherein the body defines a number of openings in each of the top andbottom bearing surfaces in communication with the hollow interior. 19.The method of claim 16, wherein the fusion cage includes a blunt nose atthe leading end of the fusion cage, the blunt nose including a recessextending from the leading end into the nose towards the trailing end,the nose further including a hole formed in said recess communicatingwith the hollow interior.
 20. The method of claim 16, further comprisingplacing bone growth material in said hollow interior.
 21. The method ofclaim 16, wherein distracting the disc space includes distracting thedisc space with a distractor having a body portion with an upper surfaceand a lower surface and a distraction height therebetween, saiddistraction height corresponding to the desired disc space heightbetween adjacent vertebral endplates.
 22. The method of claim 21,wherein said height at said leading end of said fusion cage is the sameas said distraction height of said distractor.
 23. The method of claim16, further comprising: providing a second fusion cage identical to thefusion cage; and inserting the second fusion cage in the disc space in asecond location spaced from the fusion cage.
 24. The method of claim 23,further comprising: connecting a spacer to the fusion cage to secure thespacer to the fusion cage, the spacer including a spacing member with aguide surface positionable along the fusion cage when the spacer isconnected thereto; and guiding insertion of the second fusion cage alongthe guide surface and into the second location in the disc space. 25.The method of claim 16, wherein the fusion cage includes threadsextending outwardly from the body, the threads engaging the corticalbone of the vertebral end plates.
 26. The method of claim 25, whereinthe threads include a swept back profile toward the trailing end of thefusion cage.
 27. The method of claim 26, wherein the threads increase indepth from the leading end to the trailing end of the fusion cage. 28.The method of claim 26, wherein the threads include: a crest; a convexleading wall extending between the body and the crest; and a concavetrailing wall extending between the body and the crest.